We studied the effects of antioxidants from chocolate, cacao liquor polyphenol (CLP), on human immune functions in vitro. CLP is an enriched polyphenol fraction purified from cacao liquor that is a major component of chocolate. It has been shown that polyphenols have antioxidant activity, and reactive oxygen species (ROS) are involved in immune responses. CLP inhibited both hydrogen peroxide and superoxide anion, typical ROS, production by phorbol myristate acetate-activated granulocytes. CLP also inhibited menadione-induced production of both hydrogen peroxide and superoxide anion in normal human peripheral blood lymphocytes (PBL). CLP treatment of normal PBL in vitro inhibited mitogen-induced proliferation of T cells and polyclonal Ig production by B cells in a dose-dependent manner. CLP treatment inhibited both IL-2 mRNA expression of and IL-2 secretion by T cells. These results suggest that antioxidant CLP has immunoregulatory effects.